Reflex velocity modulated discharge device



Sept. 1 1953 E. G. LINDER REFLEX VELOCITY MODULATED DISCHARGE DEVICE 2 Sheets-Sheet 2 Filed Nov. 22, 1949 INVENTOF Ernesifl index" ORNEY Patented Sept. 1, 1953 REFLEX VELOCITY MODULATED- DISCHARGE DEVICE Ernest G. hinder, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 22, 1949., Serial No. 128,690

This invention relates to improvements in velocity modulated discharge devices of the reflex type. More particularly it relates to improvement therein for reducing hysteresis or space charge efiects or both by arrangements including the use of magnetic reflection.

As is known, this is a type of device in which an electron gun projects a beam of electrons through a pair of opposed apertures of a cavity resonator along electric force lines set up between the apertures by radio frequency oscillations in the resonator. In the positive and negative halves of each radio frequency cycle, any electrons which are instantaneously passing through the resonator will be respectively accelerated and decelerated, the beam thus becoming velocity modulated. The velocity modulated electrons then move into a drift space against a retarding electric field provided by an appropriately polarized reflector electrode. of accelerated electrons will penetrate this field more deeply than the decelerated group which follows it so that by the time that they both Each group 15 Claims. (Cl. 3155) have come to a halt and have started back toward the exit aperture of the resonator the former will have fallen behind the latter in the stream but will be progressively catching up to it. All of this occurs with such timing that (1) good bunching of the beam will have been attained in the time it takes an average electron to get back to the exit aperture, and (2) the electrons comprising each bunch will pass back through the resonator n+% periods later than they were projected through it from the gun where n is any integer. Accordingly, each bunch of electrons will be retarded by the electric field which it will encounter between the exit and entrance apertures with the result that part of the kinetic energy of the electrons will be given up to increase the radio frequency energy in the resonator.

Two diiiiculties have been encountered in the prior art of. these devices. One difiiculty arises from the fact that the direction of drift of the electrons is reversed by bringing them to a halt and then accelerating them back in the opposite direction from a standstill. The resulting congestion of electrons in front of the reflector causes a great increase in the space charge of the beam and therefore in its tendency to spread. This has increased the necessity of using beams of limited electron densities to prevent the returning bunches from becoming larger in size than the resonator apertures. This difiiculty cannot be satisfactorily avoided by using apertures of increased sizes as this will entail a number of known undesirable effects. The other difficulty arises from the fact that insofar as their potentials with respect to the cavity are concerned, the cathode and the reflector both act as reflectors so that a bunch of electrons which has returned through the interaction gap is likely to be turned about and accelerated back through the gap in the incident or original direction. Since it is the common practice to place the cathode of a discharge device of this type close to the entrance aperture of the interaction gap a bunch of electrons returning toward the oathode does not have a suflicient drift period to become debunched. Because of this, the electrons which are reflected by the cathode will often still be grouped as discrete bunches, the phases of which cannot be readily controlled or predicted.

Accordingly, it has been desirable for some time to devise improvements in velocity modulated discharge devices of the reflex type whereby space charge efiects in the drift space can be overcome or reduced to improve their performance with beams of limited electron densities and to make possible the use of beams having larger electron densities and whereby hysteresis elfects can be avoided.

Accordingly, it is an object of the present invention to devise an improved velocity modulated discharge device of the reflex type in which the electron beam is less subject to spreading in the drift space due to its electron space charge.

It is a further object of the present invention to devise a discharge device of the kind mentioned above in which a means is provided for opposing the electronspace charge of the beam so that it may be of very greatly increased density without entailing excessive spreading of the bunches.

It is a further object of the present invention to devise an improved velocity modulated discharge device of the reflex type in which the electrons are not substantially decelerated or brought to a standstill in the drift space whereby their space charge is not increased in that region.

It is a further object of the present invention to devise an improved velocity modulated discharge device ofthe reflex type in which the electrons are not substantially decelerated in the driftspace and in which their negative space charge is effectively overcome therein, whereby a very high-density beam may be employed without excessive spreading of the bunches.

It is a further obj'ectofthe present invention to devise a velocity modulated discharge device of the reflex type which is improved in such a manner that a bunch of electron which has passed through the interaction gap in the direction opposite to the incident direction will not be caused to again pass through it in the incident direction, at least not as a discrete bunch.

It is a further object of the present invention to devise an improved velocity modulated dis charge device of the reflex type in which electrons, after moving into the drift space from the interaction gap of the resonator, will have their paths turned about by magnetic means in order to cause them to move back through said gap without either substantially decelerating them or bringing them to a halt.

It is a further object of the present invention to devise a velocity modulated discharge device of the reflex type which employs a magnetic means for reversing the direction of drift of electrons moving away from the resonator so that the drift space need not contain any reflecting electric field and hence may serve as an ion-trapping region in which the space charge of the electrons may be neutralized by that of positive ions to reduce or eliminate spreading of the beam.

Other objects, features and advantages of the present invention will be apparent to those skilled in the art from the following description of a number of illustrative embodiments of the invention and from the drawing in which:

Figure 1 is a diagrammatic representation of a velocity modulated discharge device of the prior art which may be helpful in explaining the above-mentioned difficulties of the prior art;

Figure 2 represents a longitudinal sectional view of one embodiment of the present invention;

Figure 3 represents a longitudinal sectional view of another embodiment of the present invention;

. Figure 4 represents a transverse sectional view of the embodiment of Fig. 3, the section being taken along the line 4--4 of Fig. 3;

Figure 5 represents a longitudinal sectional view of a modified form of the invention as shown in Fig. 3;

Figures 6 and '7 represent cross sectional views of the embodiment of Fig. 5 taken at points indicated by the lines 8-6 and 1-1 respectively; and

Figure 8 represents a longitudinal sectional view of another embodiment of the present invention.

- The schematic representation of Fig. 1 shows a cavity resonator I0 having an interaction gap through which electrons from cathode I2 are drawn in an incident direction to the right in the drawing and through gap which they are thereafter reflected, due to the negative potential of reflector I3, in a return direction to the left in the drawing. The resonator I0 may be provided with a suitable output means such as that shown at I in Fig. 2. In a region between the exit side of the interaction gap II and the surface of the reflector I3 the electrons are retarded to zero axial velocity and then repelled back through the interaction gap. Thus the stream of electrons is either moving very slowly or is at a standstill for an appreciable part of its transit time. crowded together in a region in front of the reflector I3 wherein their negative space charge reaches a very high value. This increases the tendency of the stream to spread so that when the bunches reach the interaction gap, moving As a result of this, the electrons become .4 in the return direction, they will be too large for all of the electrons to pass through the exit aperture. To avoid this, it is the common practice to employ rather low density beams. It will be noted that the cathode I2 is positioned closely adjacent to the entrance side of the interaction gap I I. The purpose of this is to prevent excessive spreading of the electrons as they are accelerated from the emissive coating I4 toward the entrance side of the cavity I 0. Because of the smallness of the space between the cathode and the interaction gap II, the bunched electrons which are reflected through the gap do not have time to become debunched as they approach the cathode and are reflected again by its low potential to make a second trip through the interaction gap in the original direction. The result is that some undesirably reflected electrons pass through the interaction gap as bunches and as such have phase relationships with the oscillations in the resonator such as to produce the well known hysteresis effects. For example, if a device of this type is frequency modulated there will be abrupt discontinuity in its frequency range of operation.

The improved device shown in Fig. 2 is arranged to overcome both of the difliculties explained above with respect to Fig. 1. In it a somewhat different beam-forming arrangement is used in that an element I 5 is added which performs with respect to the cathode I2 very much the same function as that performed by the portion of the left wall of the resonator I0 surrounding the entrance aperture of the interaction gap II. That is to say, during the operation of this device, the element I5 attracts electrons from the emissive coating I4 to accelerate them in an incident direction substantially normal to the coating, and, by collecting fringing electrons while permitting others to shoot through a beamdefining aperture therein, it forms the electrons into a pencil-like beam. The incident direction in this embodiment is not along the axis of symmetry of the cavity resonator I0 but instead at an angle thereto which has its vertex substantially at the center of the interaction gap. Beyond the interaction gap I I in the incident direction there is a means for producing a uniform magnetic field having flux lines extending perpendicularly through an hypothetical lamina in which the beam is intended to follow a turnabout path. In the Fig. 2 example, these flux lines extend perpendicular to the plane of the drawing and across a gap, not substantially thicker than this lamina, which is defined between two opposed magnetic pole pieces. One of these pole pieces, l6, appears in the background of Fig. 2, but since the plane in which this sectional view is taken extends midway between the opposed faces of the pole pieces, the cooperating pole is not shown. As is known, if an electron traveling along a straight path crosses a sharply defined boundary of, and enters into, a uniform magnetic field, it will be continuously deflected from said path in one of the two directions which are perpendicular thereto and also to the flux lines at such a rate as to follow a circular course to which said straight path is tangent at the point where it crosses said boundary. It will be deflected in a particular one of the two directions depending on the polarity of the magnetic field. In the example shown herein the exposed face of the pole piece I6 is presumed to be its south pole, i. e., the flux linesv are presumed to point downward into it so that said particulaw direction will be toward the bottom marginof, the drawing As a-result, the: circular course: of the beam .will be greater than-.180 and its cen-- ter will lie within the magneto field. Because. of this, the'electron beam. will be turned. about to such'an extent that after emerging from the magnetic field its straight return drift path will cross'its incident. path. The strength and locationof the pole pieces. It are such that the beam will crossits incidentpath ata point approximately in the center ot the interaction gap, as shown in 2'. Thus'the expended bunches after passing back through the gap l I: will drift all to one side of the: cathode l2 rather than directly back toward it. To assure that the re turning GIECtIOIIS'W-HI be. appropriately collected" andprevented from. making a return trip through the interaction gap a collector electrode [-1- is provided in the path of: the beam. This -electrode is supported on a rod to which extends through, the envelope of the tube for connection to an external source. ofdirect-po-tentiall9. If

the collector is at ground potential the elecrtons will drift toward it with the velocities: which theyretain after deceleration in the interaction gap. Since it is possible that this will result in undesired heating of the collector electrode the source of potential i9: is shown to includev a means for adjusting the potential of collector I? so that the electrons may be retarded to any desired extent and will reach the collector at reduced velocities. For certain embodiments, satisfactory operations will be obtained by simply mounting the collector electrode H on the outer left wall of the resonator and allowing it to assume the potential thereof.

If desired, the envelopeportion. 9 of the device shown inFig. 2 may be made of conductive ma-- terial so as to shield electrostatically all of the space which it surrounds. In such a case, this space will be entirely ireea-of any electric field, except for that of the electron space charge, due to: the absence of a. reflector electrode. Because of this, and as will be more fully explained below; positive ions will be entrapped in the two-way drift path of the electrons to neutralize the spacecharge thereof. and to greatly reduce or entirely eliminate electron dispersion.

Figure 3-shows an embodiment of the present invention in which a high-density type of else-- tron gun'may besuccessfully employed because of the reduction of. space. charge efiects in the drift space. This device comprises at its left end,- asshown in the drawing, a high-current ion-trap electron gun. ofa type originally disclosed in my co-pending application Ser. No. 68,605 and-alsodescribed: in co-pending applications Ser. No. 122',5 16- and Ser. No. 124,810 which are assigned to the assignee of the present application. This gun includes a cathode 20 which has the shapeoi a segment of a sphere and is supported inside anenvelope portion 2| with concave side facing: toward one side thereof. The cathode is provided with a heater 22 which may be any one oi a number of suitable types, which is supplied with heater current through a pair of leads 23 and carries an emissive coating 2 on its concave side. The coating 24 facestoward an ionization chamber 25 which is. sealed to the envelope portion 2| and through which a conical beam of electrons represented. at 26 is to be directed. To the .end that this. beam may enter thechamber 2'5 through oneof its sides while at the same time that side can serve as an electrostatic shield, its wall 21 is formed-with anelectron-permeable central. portion, e. g., a convex grid 28 ot wovenwire mesh or similar open structure; through which. large numbers of electrons may freely pass. In order that equipotential surfaces to be established between the concave coating 24- and the convex grid 28 will. be spherically concentric with said coating the grid is formed in the shape of a segment of a sphere which-is concentric with the coating. To permit electrons to pass fromthe chamber 25 on their way to the interaction gap I l of the resonant cavity H) and to the drift space beyond it a small central orifice. 29 is formed in the center of a wall 30 of the chamber 25 which lies opposite to the wall 21. The; orifice" 2-9 is concentric about the axis of symmetry of the. device and-islocated at a point a little to theleft of the point at which the convergent; electron paths of the conical beam 26 tend to. cross one another. It is desirable in the present discharge device to act upon the convergent paths of the electrons produced in the ion-trap gun so that they will become parallel paths in the interaction gap H and the drift space therebeyond. If this is not done, different electrons will be turned about by different amounts, some by more than and some by less. In order to do this", there is provided a predetermined zone betweenthe exit side of the orifice 2s: and the entrance aperture of the interaction gap- II: in Which ionsweeping fields are established. to eliminate and prevent space charge neutralization. In this zone the mutual repulsion of the electrons will cancel out the radial component of force dueto the convergent velocities of the electrons. so: that r they will be moving along parallel pathsv by the time they enter the interaction space I l. Therebeyond, as will be seen, space charge neutralization will be re-established so that spreading of the electron beam will be prevented. The means for setting up sweeping fields. in said zone con-- sists of an electrode 3i which may have very much the same shape as the wall 3110f the chain-- ber 25 and which is insulatingly sealed into the envelope of the device. When this electrode is connected to a. source of potential having a. different value from the potential. of the chamber 2.5, a sweeping electrostatic field will be established in a zone between it and the wall 30., for example, if it is connected to a source of potential lower than that of the grid 28, the field be tween the wall 3! and the electrode 3 will-have little retarding efifect on the fast-movingelectrons but willdraw the relatively static ions out of the b.eam,--leaving its space charge unneutral ized. The cavity resonator Ill is positioned to the right of the element 31. If desired, this device. may be modified by making the insulating vacuum-tightseal on the right hand side of the element 3! of very thin material having, a high dielectric. constant to provide a low radio frequency impedance in combination with directcurrent. isolation between it and the right edge of the'cyli-ndrical side walls of the resonator l0 and the left hand wall of. the resonator may .be,

eliminated so that the element 31 will effectively act as the left hand wall of the cavity resonator it. Such an arrangement would probably tend to lower the Q of the resonator and therefore it is. not preferred. The wall 8 of the resonator it which is farthest from the cathode has an exitsaperture which on its side in the above-mentioned direction-of-turn, i. e., onthe bottom side of the aperture as it. is shownin Figs. 3 and 4, extends beyond the perimeter of the corresponding sideof the entrance aperture of the resonator. This is necessary in this embodiment since the beam of electrons enters the boundary of the magnetic field at right angles thereto. The result of this is that the circular course of each electron will be for only one-half of a circle, 180, so that each electron will return toward the cathode over a path which is parallel to and displaced somewhat to one side of its incident path, rather than over a path which crosses its incident path Within the interaction gap. By using a very dense magnetic field the diameters of the semi-circular courses of the electrons may all be made quite small, with minor variations from one to another due to their different velocities, so that the displacement of the bottom edge of the exit aperture does not have to be very great in order to clear the bottom edges of the returning bunches.

Due to the curvature of the emissive coating 24 and to the electrostatic field provided between it and the convex grid 28, the emitted electrons will tend to converge toward the common center of curvature of these elements. As is known, ordinarily the sharpness of focus of an electron beam is adversely affected by the mutual repulsion of its electrons, i, e., by space charge effects, particularly if, as is likely for the gun shown herein, it is a high-density low-velocity beam. However, as is explained in the above-mentioned co-pending applications, it is possible to cause entrapment of positive ions within the conical beam so as to neutralize its electron space charge to permit precise focusing with a sharp apex at the center of curvature of the cathode. The requirements for positive ion entrapment are (l) a field-free region in which cumulative entrapment of ions can occur and (2) gas molecules in the paths of the moving electrons to provide the ions. The relatively very small number of air molecules comprising the residual gas which remains after normal evacuation will suffice for an adequate supply of ions because, as will be seen, ions will be trapped at a higher rate than they will be lost. One factor responsible for this is that a relatively considerable number of the gas molecules will be ionized by the very large electron current provided by the type of cathode shown herein.

Accordingly, in the operation of the device of Fig. 3 a very dense stream of electrons will be projected through the interaction gap II.

In its magnetic reflection arrangement, the Fig. 3 embodiment includes means for adjusting the steady-state density of the magnetic flux produced in the turn-about region and means for adding a varying component to the flux to modulate the radio frequency energy produced by this device. In Fig. 3 the source of magneto-motive force is a permanent magnet 32 which is mounted outside of the envelope in a magnetic circuit 33. This circuit includes two poles 34 and 35 having opposed ends defining the turn-about gap, a thumb screw 31 for adjusting the reluctance of a portion of the flux-carrying circuit between the permanent magnet 32 and the poles 34 and 35, and an electromagnetic winding 36 for producing a varying component of the total flux density according to an input signal. In assembling an embodiment of this type the two semicircular halves of the magnetic circuit may be placed around opposite sides of the envelope portion 9 with their respective poles 34 and 35 extending through appropriate rectangular openings therein. As represented at 38 and 39 the poles are vacuum-sealed to the edges of these openings, for example with silver solder, and

thereafter the end cover 40 of the envelope portion 9 is similarly sealed to its cylindrical portion. For appropriate mechanical and magnetic interconnection of the two halves of the magnetic circuit their lower ends may be clamped together against a spacer 46 by a bolt 41, either the spacer or the bolt or both being made of magnetic material. In the operation of the device, signals applied to the winding 36 will produce variations of the fiux in the turnabout gap. Each such variation will produce corresponding variations in the turnabout times of electrons which are instantaneously moving through the magnetic field thus varying their drift times and the radio frequency output of the device. The highest modulation frequency possible by this arrangement will be determined in part by the material of which the magnetic circuit is made and also by the series impedance presented to the input signal by the winding 36. As is known, it is possible to design magnetic circuits having very high upper frequency limits through the use of such magnetic materials as sintered ferrites and by the use of a magnetic winding with very few turns, and possibly with no more than one or one-half. However, in the embodiment of Fig. 3 another modulating means is shown which has a much higher upper frequency limit. It is a modulating electrode 4| which is insulatingly supported within the back portion of the turnabout gap by a rod 42 which extends out of the envelope portion 9 through an insulating vacuum seal so that the electrode may be connected to an external signal source. Electrode 4| should be insulated from the poles 34 and 35, for example by leaving a small amount of open space on each of its sides. When a high frequency modulating signal is applied to the electrode 42, ions which, as will be explained below, are entrapped in the drift path of the electrons wherein they serve the useful purpose of neutralizing its space charge, will not be swept from their positions therein because they will not at any time be in-' fiuenced in the same direction by that signal for a long enough time to respond to it and the direction in which the signal will influence them will continuously alternate. However, the electrons, due to their small mass, will readily respond to even the highest modulating frequency with the result that radio frequency energy produced by this device will be modulated in accordance with the input signal.

In a preferred circuit arrangement for operation of the device of Fig. 3; the chamber 25, the

resonator I0, and the envelope portion 9 may all be grounded; a source of cathode heater energy 43 is connected between the heater leads 23;

the cathode 20 is connected to an adjustable source of negative potential 44; and the electrode 3| is connected to a source of potential adjustable to values slightly above and slightly below ground potential and preferably adjusted to a value below ground potential.

In the operation of the device, electrons emitted from the spherical coating 24 are accelerated in the region between this coating and the spherical grid 28 to start to follow paths which converge at the center of curvature of the cathode and to obtain velocities which may be for example of the order of 300 volts. This will carry them into the chamber 25 wherein they will tend to drift along said paths. For a short period of time at the very start of operation the space charge of the electrons will influence the directions in which they drift so as to interfere with the focus of the beam. However, while passing through the chamber 25 the electrons of the somewhat defocused beam will cause ionization of the residual gas molecules therein, and in afew microseconds enough positive :ions will be "entrapped in the beam to neutralize its negative space charge so that all of the electrons which follow will actually move along said convergent paths.

The resulting conical cloud of entrapped ions will be relatively static, whereas the electrons, due to the kinetic energies imparted to them before they enter the chamber 25, will 'shoot'throng'h the orifice 29 in a very dense stream. .Space charge neutralization of the "conical beam within the chamber 255 will be sustained even though electrons are constantly entering it on one side and leaving-it through the orifice 218, since the average number thereof which are within the chamber at any-instant will be relatively constant, and will have a total charge substantially equal to that of the relatively'static ion cloud.

Assuming that the electrode 3| is polarized a littlebelow ground potential, some of the ions will be drawn out of the conical beam zfi through the orifice 28. However, -since the sweeping "field provided by the electrodeB l is of a very low order of magnitude very little of it will penetrate into the ion entrapment chamber '25 "and therefore ions will not escape at a higher rate than'they can be replaced byneW-ionization and entrapment, this being true 'even if the deviceis under hard vacuum.

n the other hand since the ions which escape through the orifice '29 are relatively very few by comparison with the electrons which are carried through by their kinetic-energies, and since the potential of the electrode 3! will, -as has been previously mentioned, set up a sweeping field, the region of space charge neutralization will "abruptly terminate where the-electronsemerge from the chamber 2'5. As a result, in the portion of their travel in which the electrons have-emerged from the chamberbut have not yet entered the interaction gap H the electron stream will strongly tend to spread apart radially. If the non-field free region between-the chamber 2% and the resonator It :is made 'of appropriate length and the operating potential of thecathode'is of proper magnitude the convergent paths of the electrons will be transformed intosubstantially parallel paths in this region. Beyond this region the electrons will be velocity modulated in the interaction gap 11 and will enter thed-riit space within the envelope portion 9. In :moving to the right toward the turnabout gap each group of ac-celeratedelectrons tend to catch up with the next preceding group of decelerated electrons, which at the same time will tend to fall back to meet it. This progressive bunohingwill be suspended when the electrons are in the turnabout .-'gap, the reason ior=this being that, as-can bemathematicallydemonstrated, the transit time of .an electron in such a magnetic field is independent of its velocity, i. e., two electrons which simultaneously :enter the field at different speeds will travel semicircular paths of difierent diameters with the larger semicircle being traversed at a so much higher rate by the faster electron that the two electrons will emerge at the same time. However, the bunching will .be resumed as thejgroupsof fast and slow electrons move back toward the interaction gap. Thusrit is seen that the bunchi-ngprocess will be of the type occurring in a two-cavity velocity modulated electron arson-coo 1-0 discharge device rather than in prior devices of the reflex type.

During their transit to the'left through the interaction gap the bunches of electrons will give up energy to the resonator It in the usual manher. It will be noted that some of the electrons which are able to pass through the exit aperture because of the fact that it is somewhat larger than the entrance aperture will not be able to pass through the entrance aperture and therefore will be collected on the inside surface of the left walloi the resonator. This is desirable since these electrons will not be able to contribute to hysteresis effects. Moreover, in the present device it is not likely that any hysteresis-effects will be produced even by the electrons which do pass back through the'entrance aperture. There are several reasons for this. As mentioned above, there can be no space charge neutralization in the :zone near the electrode 3!. "Thus the bunches moving to the left therein will be free torspread and-somezof their electrons will there- :fore be collected on .the outside surface of the wall 30. The remaining electrons which pass through the orifice :2-9 will have a long path .of free drift before they emerge from the grid 28 into the retarding field between it and the -cath-' ode 26.. Because of this very complete debunc'hing can take place and any electrons reflected back into the beam 126 will simply augment its steady current supply in the incident direction.

The embodiment of Fig. 5, is a modification of that of Fig. 3. Thisembodiment is shown herein to. illustrate certain principles which should be considered in practising the present invention. One of these is that the .turnabout gap should be very narrow so. that the fringing portion .of the magnetic field at its sharply delineated left boundary will havea minimum of curvature. To realize this .to the greatest extent a ribbon shaped beam is preferable, rather than one which has a circular cross section. The second principle is that there should be a minimum disparity between the sizes of the entrance and exit apertures of the resonator. Accordingly, since the least increase in size .of the exit aperture will be 00- casioned if the required offset of one of its edges is in the direction of the width axis of a beam which is very much wider than it is-thick, itwould appear that there is a second reason for using a ribbon shaped beam. Accordingly, the embodiment of Fig. =5 employs --a modified electron gun. it the cathode 59 may be constructed to have the shape of a'narrow slice taken out of the "central portion of the spherical cathode -26 of Fig. 3, e. g., by making two "parallel :cuts therethrougheach parallel to -and on an opposite side of the longest chord thereof at a short distance therefrom.- Becausecf this the grid 57 -need'not comprise as large -an areaof the wall-'27 of the ,cham-- ber 25" 'butmayhave the form of a'similar segment taken from the center of the spherical grid 28 of Fig. 8. because of these modifications of the gun, the orifice 29 should have a-rectangular s'hapeasillustrated inFigJ'Y, and the entrance and exit apertures of the .cavityjlll should be cor-.

respondingly modified.

The operation or the 'embodimentof Fig. 5 is type known as a Heil-gun, and is described by one Adolph Grunbaum in an article entitled German wartime research and development in klystrons which is listed in the U. S. Dept. of Commerce Bibliography of Scientific and Industrial Reports, vol. IV, No. 9, February 28, 1947, as

Like an ion-trap gun, the Hell-gun comprises a large-area concave emissive surface as shown at 6| in Fig. 8. The emission from this surface is drawn in a dense stream through a funnel-like accelerating electrode 62 which under typical operatin conditions for this gun would be above the potential of the cathode by an amount, such as one thousand volts, which is greater than a characteristic potential difference between the cathode 20 and the grid 28 of the embodiment of Fig. 3. Since the electrons passing out of the funnel-like electrode 62 are travelling in substantially parallel paths it is unnecessary in the embodiment of Fig. 8 toemploy a zone between the output orifice of the gun and the entrance aperture of the interaction gap H in which space charge neutralization is prevented. Instead, the embodiment of Fig. 8 is purposely arranged so that a shielded region free of static electrical fields exist along the entire beam path from the electrode 62 to the magnet poles 34 and 35 and back through the resonator 10. Accordingly there will be no ion-sweeping fields in this region since the radio frequency fields which exist within the resonator IQ and in particular across its interaction gap will be ineffective to sweep ions out of the beam path for reasons which are apparent from the foregoing. Of course it is assumed that, as in Figs. 2, 3 and 5, the envelope portion 9 is an electrostatic shield for the region which it surrounds.

Accordingly, the operation of the device of Fig. 8 is substantially the same as that for the embodiment of Fig. 2. Likewise, the operation of the modulating means shown in Fig. 8 is similar to that shown in the Fig. 3 embodiment.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What I claim as new is:

1. A velocity modulated electron discharge device of the reflex type comprising: a vacuum envelope containing an electron gun for projecting a substantially continuous beam of electrons along an axis at a predetermined velocity; resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding said axis and spaced from the other defining a capacitive interaction gap; means adjacent the aperture on the far side of the gap from the gun for producing a substantially uniform magnetic field crosswise to said axis with a sharply defined substantially planar boundary on its side nearest said gun, said field having the polarity and flux density which for said velocity and the angle at which said axis traverses said boundary cause it to guide the electrons along at least 180 of a circular path within the field to redirect them onto a return path which extends back through said gap and substantially coincides with a portion of the path of the incident beam therewithin; and a modulating electrode beyond the turnabout path of the electrons for electrically influencing the movement of electrons therein in response to an externally applied modulating signal to modulate the radio frequency energy produced by the device.

2. A velocity modulated electron discharge device of the reflex type comprising: a vacuum envelope containing an electron gun for projecting a substantially continuous beam of electrons along an axis at a predetermined velocity; resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding said axis and spaced from the other, defining a capacitive interaction gap; and means adjacent the aperture on the far side of the gap from the gun for producing a substantially uniform magnetic field crosswise to said axis with a sharply defined substantially planar boundary on its side nearest said gun and in which the angle between said axis and said planar boundary is less than said field having the polarity and flux density which for said velocity and the angle at which said axis traverses said boundary cause it to guide the electrons back toward said interaction gap in a. direction to cross said axis in said gap.

3. A device as in claim 1 which also comprises means providing a space free of static electric fields between said interaction gap and said magnetic field and in the field itself, to produce ion trapping in said space for neutralizing the space charge of the electrons.

4. A velocity modulated electron discharge device as in claim 1, wherein said electron gun comprises a large-area concave cathode for providing a copious diffuse supply of electrons along paths which tend to converge toward said axis in a focusing region and means in said region for opposing the space charge of the electrons therein to permit them to converge into a dense stream of small cross sectional area.

5. A velocity modulated discharge device of the reflex type comprising: an electron gun for projecting a dense stream of electrons along a predetermined axis at a predetermined velocity, said gun including a large-area emissive surface formed with a concave configuration having a focal point and mounted with said point on said axis, means adjacent said surface for accelerating electrons from said surface in directions toward said focal point, an ion entrapment chamber between said surface and said focal point including a first wall on its side toward said surface having a relatively large area in the paths of said electrons which is electron permeable but effective as an electrostatic shield and a second wall on its side farthest from said surface having a relatively very small beam-exit orifice con-- centric with said axis, and a gaseous medium within said chamber whereby molecules of said medium will be ionized and entrapped by electrons wthin said chamber to neutralize their negative space charge and the electrons will substantially follow said convergent paths, said focal point being beyond said orifice along said axis; electrode means positioned beyond saidsecond wall from said chamber and responsive to an applied direct potential to establish ion sweeping fields in a zone which is located along,

said axis beyond said orifice and includes said focal point for effectuating space charge forces within the beam to convert said convergent paths into substantially parallel paths; resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding said axis and spaced from the other defining a capacitive interaction gap about a.

portion of said axis near said focal point; and

means adjacent the aperture on the far side of the gap from said gun for producing a substantially uniform magnetic field crosswise to said axis with a sharply defined substantially planar boundary on its side nearest said gun, said field having the polarity and flux density which for said velocity and the angle at which said axis traverses said boundary cause it to guide the electrons along at least 180 of a circular path within the field to redirect them onto a return path which extends back through said gap and substantially coincides with a portion of the path of the incident beam therewithin.

6. A device as in claim in which the ion entrapment chamber, as measured along said axis, is substantially as long as the distance from said interaction gap to said planar boundary and back.

7. A velocity modulated electron discharge de vice of the reflex type comprising a vacuum envelope containing: and electron gun for projecting a stream of electrons along a predetermined axis at a predetermined velocity, resonator means including a pair of opposed conductive walls each having an aperture surrounding said axis and spaced from the other defining a capacitive interaction gap, magnetic means having a pair of spaced magnetic poles adjacent the aperture on the far side of said gap from said gun for producing between the poles a magnetic field transverse to said axis, said poles having coextensive closely-spaced edges on their sides toward the gun causing said field to have a substantially planar boundary at the point of where it is first traversed by said axis, said field having the polarity and fiux density which for said velocity and the angle at which said axis traverses said boundary cause it to guide the electrons along at least 180 of a circular path between the poles to redirect them onto a return path which extends back through said gap and substantially coincides with a portion of the path of the incident beam therewithin, and a modulating electrode beyond the turnabout region of the electrons for electrically influencing the movement of electrons therein in response to an externally applied modulating signal to modulate radio frequency energy produced by the device.

8. A velocity modulated electron discharge device of the refiex type comprising a vacuum envelope containing: an electron gun for projecting a substantially continuous beam of electrons along an axis at a predetermined velocity; resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding said axis and spaced from the other defining a capacitive interaction gap; means on the far side of said gap from said electron gun for producing a substantially uniform magentic field crosswise to said axis with a sharply defined substantially planar boundary on its side nearest said gun, said field having the polarity and flux density which for said velocity and the angle at which said axis traverses said boundary cause it to guide the electrons along at least 180 of a circular path within the field to redirect them onto a return path which extends back through said gap and substantially coincides with a portion of the path of the incident beam therewithin; and means responsive to a modulating signal to combine with said magnetic field a component which varies in accordance with the signal for modulating the radio frequency energy produced by the device.

14 9. A velocity modulated electron discharge device of the reflex type comprising: an electron gun for projecting a substantially continuous beam of electrons along an axis at a predetermined velocity; a pair of opposed conductive Walls each having an electron-permeable area surrounding said axis and spaced from the other to define an interaction gap; means positioned on the far side of said gap from said gun for producing a substantially uniform magnetic field crosswise to said axis in a region spaced from said gap and adapted to deflect the electrons along a circular path within said field and redirect them onto a return path which extends back through said gap; a hollow conducting member completely enclosing said region and the space between said interaction gap and said region; and a gaseous medium in said member to produce ion trapping therein for neutralizing the space charge of the electrons.

10. A velocity modulated electron discharge device of the reflex type comprising: an electron gun for projecting a substantially continuous beam of electrons along an axis at a predetermined velocity; a pair of opposed conductive walls each having an electron-permeable area surrounding said axis and spaced from the other to define an interaction gap; means positioned on the far side of said gap from said gun for producing a substantially uniform magnetic field crosswise to said axis in a region spaced from said gap, said field having the polarity and flux density which for said velocity and the angle at which said axis traverses the boundary of said field cause it to guide the electrons back toward said interaction gap in a direction to cross said axis in said gap; a hollow concentric member completely enclosing said region and the space between said interaction gap and said region, and a gaseous medium in said member to produce ion trapping therein for neutralizing the space charge of the electrons.

11. A device as in claim 10, including a collector electrode on the gun side of said gap and in said return path.

12. A device as in claim 9, in which said electron gun comprises a large-area cathode and means for focusing the electrons from said cathode into a dense beam of small cross-sectional area at said gap.

13. A device as in claim 9, including means responsive to a modulating signal for varying the electron paths in said space, to modulate the radio frequency energy produced by said device.

14. A device as in claim 13, wherein said path varying means comprises means for varying the fiux density of said magnetic field.

15. A device as in claim 13, wherein said path varying means comprises a modulating electrode disposed in said magnetic field and on said axis.

ERNEST G. LINDER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,060,770 Hansell Nov. 10, 1936 2,272,165 Varian et al. Feb. 3, 1942 2,306,875 Fremlin Dec. 29, 1942 2,457,495 Rochester Dec. 28, 1948 2,468,152 Woodyard Apr. 26, 1949 2,470,856 Kusch May 24, 1949 

